A thorough understanding of the plasma and intracellular levels of antiretroviral drugs and their metabolites is essential in order to judge a compound's actual potential in humans. In a study to determine the intracellular half-life of the antiretroviral compounds GS-0397 and GS-1278, both were labeled with 14C and injected subcutaneously into macaques. Blood and lymph node samples were collected at various time intervals and the concentrations of the radioactive material in the plasma, blood cells, and lymph nodes were determined by quantitative methods.